1. Field of the Invention
This invention relates to global positioning system based navigation techniques for developing position and velocity data required for autonomous operation of satellites incorporating on-board, event-based command systems.
2. Discussion of the Related Art
Spacecraft tracking and navigation began when The Johns Hopkins University Applied Physics Laboratory determined Sputnik""s orbital parameters as a function of the doppler shift of an on-board radio beacon. That was in 1957. Soon thereafter, that organization devised and developed the Navy Transit Satellite Navigation System which provided global navigation services for the military and civilian community for over 35 years. The xe2x80x9cTRANSITxe2x80x9d system used a network of low earth orbit satellites and therefore it was unacceptable for most spacecraft navigation applications.
A limited number of experimental systems developed by the Applied Physics Laboratory (APL), NASA and the U.S. Air Force provided semi-autonomous means for orbit determination and prediction. The experimental system provided ground-based orbit determination for spacecraft through the use of complex ground stations and coherent transponders that reside on the spacecraft to be tracked. The French National Space Agency has operated a similar system, DORIS, since the early 1990s. However, the high cost of operating such systems has forced spacecraft operators to find other tracking methods.
Over the last three decades, APL and other organizations have demonstrated the feasibility of using GPS for positioning satellites and other high dynamic platforms. For over 20 years, the APL developed SATRACK system has utilized GPS translators and ground-based signal processing systems for trajectory reconstruction and guidance system evaluation of Navy Trident missiles. The first GPS-based navigation of a satellite occurred on Transat, an APL spacecraft launched in 1978 and operated for over 10 years. Transat was a Transit navigation satellite with an on-board GPS translator.
Autonomous positioning of satellites using space borne GPS receivers was first demonstrated in the early 1980s when APL developed and flew four GPSPAC systems, and in the early to mid 1990s when NASA and the Jet Propulsion Laboratory used GPS receivers on the TOPEX/Poseidon spacecraft. More recently, other programs have adopted the use of GPS-based navigation systems for spacecraft. For instance, U.S. Pat. No. 5,109,346 for xe2x80x9cAutonomous Spacecraft Navigation Systemxe2x80x9d issued to J. Wertz on Apr. 28, 1992 describes a system using onboard observations of the earth, sun and moon to determine spacecraft attitude, instantaneous position and orbit based on multiple position estimates. Position and orbit data are derived by multiple deterministic solutions, including some that employ star sensors and gyros, and the multiple solutions are accumulated in a Kalman filter to provide continuous estimates of position and orbit for use when the sun or moon is not visible.
Developing totally autonomous satellites results in large cost savings because of the elimination of ground support stations. In addition, ground supported satellites are subject to ongoing maintenance costs and are prone to serious malfunction if ground support stations are damaged or if control signal transmissions are interfered with.
A primary objective of the present invention is to provide a satellite for collecting data which utilizes on-board event-based command techniques relying on a navigation system which uses an extended Kalman filter to process data derived from a plurality of global positioning satellites.
A further objective of the present invention is to provide an orbital platform for collecting data which utilizes an autonomous navigation system incorporating an application specific integrated circuit for processing signals from global positioning satellites in concert with an extended Kalman filter.
Another objective is to provide an autonomous, on-board, event-based command system responsive to a GPS based navigation system for controlling data collection functions on-board low-earth-orbit and medium-earth-orbit satellites.
A further objective of the invention is to provide an extended Kalman filter combined with an application specific integrated circuit for determining continuous real time earth-sun vector data, from a GPS constellation comprised of at least four satellites.
A still further objective is to provide an autonomous navigation system whose primary input data is derived from at least four satellites of a GPS constellation.
An objective of the invention is to provide an extended Kalman filter for interpreting data, from a GPS constellation comprised of at least four satellites and predicting ground contact event times therefrom.
Another objective is to provide an application specific integrated circuit for processing data required for tracking global positioning system satellites.
A further objective of the invention is to provide an extended Kalman filter combined with an application specific integrated circuit for interpreting data, from a GPS constellation comprised of at least four satellites, as continuous real-time orbital platform position and velocity.
A still further objective of the invention is to provide an extended Kalman filter combined with an application specific integrated circuit for propagating state vectors to allow estimating future orbital platform position from a GPS constellation comprised of at least four satellites.
The primary purpose of the present invention is to provide an autonomous navigation system for satellites to be employed in NASA""s Solar Terrestrial Probe Program for measuring Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED). The TIMED program is a remote atmospheric remote sensing mission to study the interaction of the Sun and Earth""s atmosphere. This spacecraft serves as an orbital platform for four instruments that measure the basic state parameters and energy balance of the mesospheric, lower thermospheic, and ionospheric regions, i.e. MLTI regions of the atmosphere. The program investigates and documents the energetics of the MLTI region, i.e., its pressure, temperature, density, and wind structure and the relative importance of various sources and sinks of energy. To make the required measurements, a 625-km circular orbit inclined 74.1 degrees with a nodal regression of 7200 per year is used. On-orbit instruments augmented by an array of ground-based instruments provide the basic measurements required by the mission. The instruments require precise knowledge of position and velocity of the orbital platform to perform their mission. This is accomplished by the autonomous navigation and time keeping system which provides position, velocity, time, and earth-sun vector data and notification of defined orbital events in real-time. It also generates predictions of events such as ground stations contacts. In addition, it generates orbital element sets which are down-linked for use by ground station antenna pointing systems. The navigation system contains two processors; a tracking processor which is responsible for controlling and interacting with GPS hardware in order to obtain raw tracking data and the navigation processor which is responsible for command and telemetry handling, determination of navigation solutions, generation of tracking acquisition aids and generation of the output data products. To simplify command control, the GPS based navigation system (GNS) is used to produce accurate estimates of position, velocity, and time to an event-based command system.
The GNS uses SPS ranging signals broadcast from the constellation of GPS satellites. The SPS ranging signal, referred to as L1 is Binary Phase Shift Key (BPSK) modulated. The modulation consists of two components that are modulo-2 summed: (1) a 1.023 MHz Pseudo-Random Noise (PRN) code known as the coarse acquisition (CA) code and (2), a 50 Hz navigation message. The CA code sequence repeats every 1 ms. The GNS receiver demodulates the received code from the L1 carrier, and detects the time offset between the received and a locally generated replica of the code. The receiver also reconstructs the navigation message data.
To compute TIMED spacecraft position, velocity, and time, the GNS determines the pseudorange to four or more GPS satellites in track. The propagation time to each GPS satellite is obtained by determining the difference between transmit and receipt times of the CA code. The pseudorange to each GPS satellite is computed by multiplying each propagation time measurement by the speed of light.
The navigation message transmitted from each GPS satellite provides data which are required to support the position determination process. That includes information to determine satellite time of transmission, satellite position, satellite health, satellite clock correction, time transferred to UTC, and constellation status.
FIG. 1 illustrates the 660 kg TIMED spacecraft which is three-axis stabilized and NADIR pointing. The majority of the bus electronics are contained in an integrated electronics module (IEM). The IEM is a highly integrated system with major spacecraft bus subsystems implemented on cards in a card cage to eliminate the more common self-contained modules or xe2x80x9cblack boxesxe2x80x9d. The subsystems that populate the IEM are on cards which have common mechanical interfaces based on the Stretch SEM-E form-factor to simplify the design process. Two IBM processors are employed, configured as single-string redundant systems with a 1553 bus connecting the two.
A graphite epoxy optical bench is mounted on the spacecraft""s zenith-facing surface to provide a thermally stable mounting surface for TIMED Doppler Interferometer (TIDI) telescopes and the attitude system""s star trackers (required to satisfy TIDI attitude knowledge requirements). Two GPS Navigation System (GNS) antennas are mounted atop composite masts on the optical bench for performing on-orbit GPS attitude determination verification with a precision reference, or truth, data set.
The TIMED mission and spacecraft top-level command and control requirements are satisfied by the GNS which is designed specifically for TIMED and implemented on Application-Specific Integrated Circuits (ASIC). Although designed for TIMED, the GNS will satisfy the functional requirements that are typically placed on spaceborne autonomous navigation systems for LEO or MEO host vehicles.
The GNS includes redundant Standard Positioning Service (SPS) receiver systems with access to the GPS civilian ranging code called the coarse acquisition (CA) code that modulates the L1 (1575.42 MHz) signal. The GNS is a state-of-the-art spaceborne system optimized for autonomous on-orbit operations. The GNS is radiation tolerant, has extensive command and telemetry capability, provides access to raw and intermediate data products, supports on-orbit software reprogramming, is designed to accommodate the large doppler signal dynamic range resulting from orbital velocities, and implements robust signal acquisition, navigation, and orbit determination algorithms.